MOSCOW — In late April, a cache of letters from jailed American Paul Whelan appeared in the Michigan mailbox of his parents, the first contact Whelan's family has had with him since he was arrested four months ago and accused of espionage by Russian authorities.

The Russian state has stayed largely quiet on the situation surrounding Whelan, 49, not answering the increasingly exasperated demands of U.S. officials to present evidence for detaining the former Marine on spy charges - despite claims by the Kremlin that he was caught "red-handed."

Since he was seized by Russian security agents in his hotel room during what his family says was a personal trip to Russia, a Moscow court has prolonged his detention. The court's latest ruling dictates that Whelan stay behind bars until at least May 28.

"Russia is denying him his basic human rights and has been dragging its feet every step of the way," the chair of the House Foreign Affairs Committee, Rep. Eliot Engel, D-N.Y., said this week. "I renew my demand that the Russian government end this charade and release Paul Whelan back to his family."

The sudden arrival of about 40 letters, which Whelan penned in the early days of his imprisonment, came by regular airmail in two batches, taking his family by surprise.

"Our parents were delighted. Paul told them about his daily life, how he is living there," said David Whelan, Paul's twin brother, speaking by telephone from Canada, where he lives.

He added that Whelan seemed well, "judging by our parents' reaction."

The handwritten letters are long, wordy and full of detail. "When someone writes a letter, a certain amount of their voice appears. [Our parents] could tell when he was making a joke, and that meant a great deal to them," added David Whelan, who shared a photo of the letters on Twitter.

The envelopes have Russian stamps on the top-right corner, depicting famous Russian fortresses including the Kremlin. In the top left, Whelan has written the return address: Lefortovo prison.

A letter to an aunt in Britain also recently arrived, David Whelan added.

Paul Whelan's brother holds citizenship in four countries - Britain, Ireland, Canada and the United States.

"I'm not relying on this being a regular thing. Perhaps these letters escaped," David Whelan added.

Handwritten correspondence and the use of postal services have become something of a theme in Whelan's case. Instead of signing key documents and handing them to U.S. Embassy staff, Whelan has been forced to send a privacy act waiver and power of attorney by mail. U.S. officials were later told by Russian officials that they had been "lost" for weeks.

Whelan has not been given any of the 100-plus pieces of mail his family has sent him, according to his family.

"Why haven't Russian officials provided proof?" U.S. Embassy spokeswoman Andrea Kalan wrote on Twitter, referring to the spy claims. "Perhaps it's lost along with Paul's mail."

According to the family's lawyer, Ryan Fayhee, Whelan was finally able to sign another privacy act waiver last month, allowing consular officials to pass information between Whelan and his family.

Unlike other inmates at Lefortovo, Whelan has been denied phone calls to his family, and the letters were the first time since his arrest that they were able to get a direct sense of his well-being in his own words.

But why the letters arrived so late, or at all, remains a mystery, the latest twist in an opaque case where the regular rules of diplomacy seem to carry little weight.

Shortly after Whelan was arrested by Russian security services in late December, he was charged with espionage, a crime that carries a prison sentence of up to 20 years. Whelan's Russian lawyer has said his client was unwittingly handed a flash drive containing "state secrets."

When the U.S. ambassador to Russia, Jon Huntsman Jr., visited Whelan in prison on April 30, he was prevented from reading aloud letters from Whelan's family, or "talking about anything that actually matters," Kalan wrote on Twitter. "Consular access without being able to speak openly is frankly a joke."

Americans are not the only ones frustrated by Whelan's treatment. Members of the Public Monitoring Commission in Moscow, a body that inspects prisons, were denied a return visit after their two initial outings to the Lefortovo pretrial detention center in January, shortly after Whelan's arrest.

When they did see him, they were prevented from speaking without an interpreter, even though they have a command of English.

"There is a rule that the arrested person is supposed to sign papers saying they have read the rules of the jail. How could he read these rules if they are in Russian?" said Evgeny Enikeev, a member of the commission and a human rights activist with the Moscow Helsinki Group.

Nevertheless, Enikeev said Whelan has an English-speaking cellmate, who is around 40 years old, and was living in the recently refurbished part of the jail, meaning he has been spared the hole-in-the-ground toilets and grim outdated kitchen that shaped so many inmates' experiences at Lefortovo.

"Several inmates have told us that the quality of food has become much better," Enikeev said. "Simple food without anything fancy."

Although there is still no door on the toilet of Whelan's cell, there are now relatively high partitions that create some privacy, he said. For exercise, inmates are led in pairs to an open-air cell on the roof of the prison measuring 30-by-50 feet.

Enikeev fears Whelan could join other long-timers in Lefortovo, even though the prison is by definition for those awaiting sentencing: "If our system sinks its teeth into someone, he will not be released."